In many applications, in particular for high speed optical communication systems, a modulated light wave is used to carry digital information from a sender to a receiver. In many such systems, the modulation may be phase and/or amplitude modulation. Examples include phase shift keying modulation techniques, such as Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), and Quadrature Amplitude Modulation (QAM) techniques, such as QAM8, QAM16 and QAM64.
In order to achieve such modulation of a carrier light wave, it is known to split the carrier light wave using a splitter, and to recombine the carrier light wave in a combiner after a relative phase shift of the different light paths between the splitter and combiner. The phase shift can for instance be achieved using electrodes attached to each path, to each of which electrodes a variable electrical signal can be applied so that the refractive index of the path wave guide material changes, forming a Mach-Zehnder interferometer. Such variable phase shift can be combined with a predetermined fixed phase shift for each waveguide. This way, each symbol can be modulated as a unique combination of total phase shifts along each path. A modulator in which a first parallel-coupled Mach-Zehnder modulator (MZM) controls the imaginary part of the electromagnetic field (Q value) and a second parallel-coupled MZM controls the corresponding real part (I value) is called an IQ modulator.
WO 2011022308 A2 discloses using an MZM, yielding two paths, or two parallel-coupled child MZMs on one respective path of a parent MZM, yielding in total four paths, with variable-current electrodes on each path, for such modulation.
Known two-armed structures offers limited possibilities to achieve advanced modulation formats, such as higher-order QAM formats, without using complicated drive electronics. In many cases, it is preferred to use electrode voltages that have predetermined, fixed values, preferably at a few, most preferably only two, different voltage values. This makes the control electronics fast and simple, which is preferable for high bitrates.
Yossef Ehrlichman, et. al., “A Method for Generating Arbitrary Optical Signal Constellations Using Direct Digital Drive”, JOURNAL OF LIGHT WAVE TECHNOLOGY, VOL. 29, NO. 17, Sep. 1, 2011, discloses a method for creating various such symbols using a two-armed structure.
Hyeon Yeong Choi, et. al., “A New Multi-Purpose Optical Transmitter for Higher-Order QAM Generation”, OFC/NFOEC Technical Digest, 2013, discloses a method for creating arbitrary modulated symbols using a first and a second four-arm MZM aggregate.
These methods require complicated drive electronics, making them expensive and less suitable for high-frequency applications.
Furthermore, WO 2011022308 A2, above, discloses the use of multiple, individually controlled electrode segments for each optical path in the modulator. The purpose of this is to mitigate the nonlinearity of the relation between the electrode voltage and the resulting refractive index of the waveguide material. The segments are controlled individually to select the total phase modulation applied to each path.
A conventional IQ modulator is associated with low output noise, since total or partial anti-symmetric voltage noise results in the reduction of optical phase noise. However, it is a complex structure, requiring many electrical contact points and relatively large build dimensions.
Hence, it would be desirable to achieve an optical modulator capable of modulating complex symbols, such as according to various PSK and QAM modulation scheme versions, which is less complex than a full IQ modulator or the corresponding, and which does not add significant noise as compared to the conventional IQ modulator. Furthermore, it would be desired to achieve such a modulator which can be driven by a driver with only simple control logic. Preferably, such a modulator would also have high power transmission.
In particular, it would be desirable to achieve such a modulator which is suitable for use in mixed material systems, such as systems using silicon for the drive electronics and active bandgap III-V materials such as indium phosphide or gallium arsenide for the optical waveguides.
The present invention solves the above described problems.